I am currently designing a circuit where:
- An LED driven by a constant current driver shines directly into a photodiode. (I have also tried without the op amp constant current circuit.)
- The photodiode is operating in photovoltaic mode, into an op-amp.
- The output of the op-amp is directly plugged into my headphones (I should AC couple but not in the schematics).
Now, what is strange is :
- When the photodiode is covered, I can hear absolutely no noise (even when the LED is lit).
- When I let my desk lamp shine into the photodiode, I can clearly hear the 60 Hz hum. Using a membrane partially covering the photodiode, I can hear sounds made by the vibration of the membrane.
- When I approach the LED, I hear a very strong background high frequency white noise (some sort of hiss).
The LED is powered through DC (I would say reasonably clean DC), so where is this "optical noise" coming from? Is this the 1/f noise some papers talk about? Is there a driver circuit to avoid it? Alternative light sources?
First, thanks you all, it is very formative. It seems I made a bunch of mistakes:
- Made assumption about the cleanliness of my power supply. I am more used on digital design, I just sprinkled a bunch of capacitors, could not hear any noise on the sensor and called it a day. I've got a more professional supply on the way and will make sure to be more cautious with regards to decoupling capacitors
- My constant current circuit does not seems to be working properly. I need a voltage reference on the high side
- My quad op-amp should have its unused inputs tied.
I also made the formal mistake of not sufficiently relying on my scope and instead focusing on noise on the audio output. This stems from difficulty in characterising what is "problematic" noise from noise that is to be expected. If that is useful, here is a fft on the output;
In the end here what seemed to be the issue :
- A driver circuit that was unstable with a single supply op amp. I am not totally sure why, but switching to dual supply solved some of the issue, even when I made sure not to run the op amp into the rails (it might also be that the dual supply op amp was less noise prone). That driving circuitry might also not be the best choice (also talked about here, and described as "prone to oscillate").
- A bad power supply that added to the noise
The very same circuit with a dual supply was up to my expectations. I've still got to get a better grasp of op amps functionning (I'm working my way through the Art of Electronics, but slowly), but one thing that really limited me was my ability to characterize the noise with a scope. Audible difference did not show up significantly on the FFT, so it would really help if I could be directed to a primer on the matter. Anyway, RusselH's answer was the most complete based on my incomplete description so I'll mark it as accepted !